This invention relates generally to optical measurement probes and, in particular, to probes which utilize a window configuration suited to on-line process control and other applications and, more particularly, to an improved technique for bonding such a window to probe body or process vessel.
Optical characterization techniques based upon the Raman effect and fluorescence have become important tools in process monitoring and control for industries involved with hazardous materials, pharmaceuticals, and so forth. In such applications, it is typical for probe having a sealed window to be introduced into the process flow, making remote connection to analytical equipment through optical fibers, for example.
Current immersion window designs used by spectroscopic analysis systems rely on several techniques for sealing the window to the probe body. The two most broad categories are elastomeric seals, which include O-rings, gaskets, adhesive, etc., and metal seals. Metal seals can be subdivided into soft solder, metal spring seals and compression bonding. In general, metal seals are the most desirable type, offering the potential of high temperature resistance, high-pressure resistance, hermetic sealing, longevity and robustness.
The brazed window seal is the most common in the spectroscopic probe industry. Shown in FIG. 1, this arrangement consists of a window 102, typically sapphire, which is brazed to the surrounding probe body 104, typically with gold or a gold alloy 106. This technique has all the advantages of metal seals listed above, plus the fact that the sealing is done to the edge of the window thus leaving the surface flush to the outside of the probe without any special forming of the window itself. The disadvantages of this approach are that thermal shocks, particularly those proceeding from cold to hot, can create large tensile stresses and crack the window. In addition, these window assemblies are expensive and time-consuming to construct.
An alternative to the brazed window is the compression bonded window shown in FIG. 2. In this case, the window 202 is tapered and is pressed into the probe body 204. The interface may or may not have a ductile metal, such as gold, to aid in the sealing. This approach is inexpensive and quick, but has the disadvantage of losing compression on the window as the probe body expands at high temperature.
In an optical measurement probe of the type which incorporates a window configuration suited to on-line process control and other applications this invention provides an improved technique for bonding such a window to probe body or process vessel. In general terms, the improvement arises through the use of a compression ring around the bonding area to maintain a consistent seal.
In terms of apparatus, a hollow probe body terminates in a distal end having a flanged portion with an inner and outer wall that preferably extends back into the body. The outer wall of the window is sealed against the inner wall of the flanged portion, with compression material urged against the outer wall of the flanged portion to pressurize the seal between the window and the inner wall of the flanged portion.
In the preferred embodiment, the compression material is ring-shaped and exhibits substantially the same thermal expansion coefficient as the window. In the case where a sapphire window is used, titanium or Invar may be used for the compression material. Invar is well know to those of skill in mechanical design as an alloy of iron and nickel, plus other elements such as silicon and manganese, to achieve a very low coefficient of thermal expansion. The window may be generally cylindrical though, in the preferred embodiments, cork-shaped or spherical window are used.